Increased running speeds of paper machines provide new problems to be solved, which problems are mostly related to the runnability of the machine. Currently web running speeds of up to about 1600 meters per minute are employed in printing-paper machines. At these speeds, the so-called closed press sections, which comprise a compact combination of press rolls arranged around a smooth-faced center roll generally operate satisfactorily.
With increasing running speeds of paper machines, the problems of runnability of a paper machine are also manifested with higher emphasis, because a web with a high water content and low strength does not endure an excessively high and sudden compression pressure impulse or the dynamic forces produced by high speeds, but web breaks and other disturbances in operation arise and cause standstills. In a modem printing-paper machine, the cost of standstill time is today about: FIM 60,000 per hour ($13,200).
Further problems manifested with increased emphasis at high speeds of paper machines, for. which problems, at least for all of them in toto, satisfactory solutions have not been found as yet, include the problems of quality related to the requirements of uniformity of the profiles of properties of the paper web both in the machine direction and in the cross direction. Uniformity of the web produced by the paper machine also affects the runnability of the entire paper machine, and it is also an important quality factor of finished paper, which is emphasized in the case of copying and printing papers with increasing speeds of copiers and printing machines and with higher requirements imposed on the uniformity of the printing result.
Recently, even speeds as high as about 40 meters per second (2400 meters per minute) have been contemplated as running speeds of paper machines. Application of running speeds as high as these, in particular in wide machines, provides ever more difficult problems to be solved of which problems, the most important ones are runnability and adequate dewatering capacity of the machine at a high speed.
With respect to the prior art related to the press section of a paper machine, reference is made to U.S. Pat. No. 5,389,205 (Pajula et al.), which is hereby incorporated by reference herein, which describes a method and device for dewatering a paper web by pressing. In this patent, a method is suggested in the manufacture of paper or board for dewatering the paper web that is being manufactured and that has been drained in the web former of the paper machine. In the disclosed method, the dewatering takes place by passing the paper web on support of fabrics that receive water through a number of successive dewatering nips so that, by the effect of the compression pressure. water is transferred out of the fiber mesh of the paper web into the spaces in the fabric that receive water and into the spaces in the hollow faces of the mobile dewatering members, such as press rolls. Further, the paper web is transferred from the forming wire onto the wire of the dryer section while constantly on support of a fabric that receives water, a transfer fabric, or any other, corresponding transfer surface as a closed draw at a speed that is higher than about 25 to about 30 meters per second. It has been considered as one of the novel aspects of this prior art method that, in the method, dewatering of the paper web is carried out by means of at least two such successive press nips of which nips at least one press nip is a so-called extended-nip zone whose length in the. machine direction is larger than about 100 mm, and the extended-nip zone is formed in connection with a mobile flexible press-band loop. The distribution of the compression pressure employed within the extended-nip press zone is regulated and/or selected both in the cross direction of the web and in the machine direction so as to set or to control the different profiles of properties of the web. Also, as the first press stage, a dewatering pressing is carried out on the web forming wire by using a press zone and a water-receiving, relatively open fabric or fabrics running through that press zone.
As known from the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against the heated cylinder faces of drying cylinders situated in two separated rows. Between the rows of drying cylinders, which are usually horizontal rows, the web may have free and unsupported draws which are susceptible to fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, has a low strength. Therefore, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders comprises only one drying wire on whose support the web runs through the entire group so that the drying wire presses the web against the heated cylinder faces of the drying cylinders and the web remains at the side of the outside curve of the reversing cylinders or rolls situated between the drying cylinders. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the loop.
With increasing running speeds of paper machines, problems of runnability have also started occurring in the area of single-wire draw, in particular in the first groups in a dryer section. In a manner known from the prior art, attempts have been made to reduce these problems by using various components of runnability, such as the Uno Run Blow Box (the current assignee's trade mark) and by replacing the lower roll by a suction roll, for example a VAC-roll. However, so far, the running speeds are not yet known up to a level at which these prior art constructions are sufficient to support the web in the beginning of the dryer section when the speeds continue to become higher.
With increasing speeds of paper machines, the runnability of a paper machine is, of course, also affected by the dryer section, whose length with the prior art multi-cylinder dryers would, at high speeds, also become intolerably long. If it is imagined that a present-day multi-cylinder dryer were used at a web speed of about 40 meters per second, it would include about 70 drying cylinders, and its length in the machine direction would be about 180 meters. In such a case, the dryer would comprise about 15 separate wire groups and a corresponding number of draws over the group gaps. It is to be assumed that, in a speed range of from about 30 to about 40 meters per second, the runnability of the prior art multi-cylinder dryers would no longer be even nearly satisfactory, but web breaks would be frequent, which would lead to a deterioration in the efficiency of the paper machine.
In a speed range of from about 30 to about 40 meters per second and at higher speeds, the prior art multi-cylinder dryers would also become uneconomical because the cost of investment of an excessively long paper machine hall would be unduly high. It can be estimated that, at present, the cost of a paper machine ball is typically about one million FIM per meter in the machine direction (about $220,000).
It is known from the prior art to use various impingement drying/through drying units for evaporation drying of a paper web, which units have been used in particular for drying tissue paper. With respect to the prior art related to this, reference is made, e.g., to U.S. Pat. Nos. 3,874,997, 3,868,780, and 5,319,863.
With respect to the prior art related to the present invention, reference is made to an article entitled "Trends in high speed machines for newsprint and groundwood papers"Pulp & Paper, Apr. 1983, pages 100-103. In this paper, among other things, a newsprint machine is described which is operated at a speed of about 1000 meters per minute and in which, in the dryer section, a web support of full width is employed without draws between the dryers. In a pre-dryer in the dryer section, vacuum boxes and vacuum rolls are arranged inside the wire in order to keep the web in contact with the belt. The web is dried in the pre-dryer in the dryer section by means of hot air to a dry solids content of from about 45% to about 50%.
With further respect to the prior art, reference is made to U.S. Pat. No. 4,361,466 which describes a method and mechanism for removing water from a web in a paper machine in which there are press members and a first dryer unit based on heating in which there is a long, continuous, endless support belt which carries the web during the first drying cycle. In the first drying cycle, the rolls and the suction zones are placed below the web and there are members that blow hot air, as well as members by whose means the air blow is directed at the web, on the first heat-treatment run or which the web is received substantially at a dry solids content of about 40% and from which the web is removed substantially at a dry solids content of about 50%. In this prior art construction, the paper. web arrives and departs as an open draw into/from the pre-drying unit.
In addition, with respect to the prior art, reference is also made to the U.S. Pat. No. 5,256,257 which describes a construction in which a non-water-receivable transfer belt runs through two press nips and transfers the web to the dryer section as a supported draw so that the web can be. heated/dried by impingement drying between the press and a group with single-wire draw.
In a manner known from the prior art, the web is passed from the press section to the dryer section so that the web has been separated from the last smooth roll in the press section and passed by means of a guide roll to the dryer section, in which case the web has had a free draw directly after the press section. This passage has proved problematic, in particular because of the increased risk of web break in this connection. In order to amend this, a closed draw has been developed from the press section to the dryer section, such a closed draw being described, for example, in U.S. Pat. No. 5,389,205 mentioned above, in which the web is passed from the press section by means of a transfer belt to a group with single-wire draw in the dryer section. As is well known to those skilled in the art, a web tightness arising in connection with an open draw improves the running quality of the web, and in closed draws of the web, attempts have been made to produce a web tightness by using a difference in speed between the different support fabrics. This has, however, produced problems because in such a case, the support fabrics are subjected to rapid wear. In paper machines having very high web running speeds an adequate web tightness has not been achieved by means of a difference in speed, in which case the web has not followed the wire in the dryer section but rather. owing to its slackness, it has caused web breaks, fluttering, and similar problems.
The wet strength and the elastic properties of a paper web depend on the dry solids contents of the web, and directly after the press section, it has been problematic to make the web sufficiently tight because the web has not been sufficiently dry thereat. For this reason, in cylinder groups with single-wire draw, which are often placed in the beginning of the dryer section, i.e., in so-called slalom-draw groups, problems have been encountered in the runnability, in particular in high-speed paper machines. As one solution, short groups of just a few cylinders have been employed in the beginning of the dryer section, so that by means of a positive difference in speed between the groups, it has been possible to maintain web tightness. This solution, however, increases the costs of investment end operation because of the increased number of wire circulations.
Moreover, when tightening of the paper by means of differences in speed between support fabrics has been employed, the paper web may be constricted unevenly, and high differences in tension applied to the web may cause problems in achievement of a sufficiently uniform quality, in particular in relation to the cross-direction profile of the paper.